1. Field of the Invention
The present invention relates to a crystallized glass to be used for a substrate for an optical filter such as a band pass filter to be used for a wavelength division multiplexing (WDM) optical communication system, and an optical filter.
2. Discussion of Background
In the WDM optical communication system, a band pass filter is required which is capable of selectively transmitting a light within a specific wavelength range, for example, a light of a specific wavelength of from 1530 to 1620 nm. As such a band pass filter, a band pass filter having a dielectric multilayer film formed on a substrate such as a glass substrate or a crystallized glass substrate, is used.
The above-mentioned dielectric multilayer film is a film in which a thin film of high refractive index dielectric such as TiO2 or Ta2O5 and a thin film of a low refractive index dielectric such as SiO2 are alternately laminated, and it is typically a multilayer film of about 100 layers.
A substrate to be used for such a band pass filter is required to have an average linear expansion coefficient αL at from −30° C. to 70° C. within a range of from 95×10−7/° C. to 130×10−7/° C. in order to compensate a change by temperature of the refractive index of the dielectric multilayer film thereby to suppress a change by temperature of the wavelength passing through the band pass filter.
A glass (“conventional glass”) having a composition comprising, as represented by mol %, 47.3% of SiO2, 24.2% of TiO2, 15.6% of Na2O, 6.3% of K2O, 4.6% of Li2O, 1.8% of BaO and 0.2% of MgO, has its αL being 101×10−7/° C., and is used as a substrate for a band pass filter.
However, a glass substrate made of such a conventional glass has had a problem that warpage of the substrate at the time of film formation is large. This is considered to be attributable to the fact that the Young's modulus (hereinafter referred to as E) of the conventional glass has a small value at a level of 83 GPa. Accordingly, a substrate having larger E is desired.
As a substrate to solve this problem, a crystallized glass substrate has been proposed which is made of a crystallized glass (hereinafter referred to as the conventional crystallized glass) having a composition comprising, as represented by mol %, 74.1% of SiO2, 4.0% of Al2O3, 0.6% of ZrO2, 1.2% of K2O, 18.1% of Li2O, 1.1% of MgO, 0.4% of ZnO and 0.4% of P2O5 and having a lithium disilicate crystal precipitated therein. This conventional crystallized glass has αL of 111×10−7/° C. and E of 96 GPa.
The crystallized glass substrate made of the conventional crystallized glass is one which solves the problems relating to αL and E, but has had a problem that adjustment of the film-forming conditions is difficult as compared with the case where a multilayer film is formed on a glass substrate made of the conventional glass.
It is an object of the present invention to provide a crystallized glass for an optical filter substrate and an optical filter, which solve the above-mentioned problems.